In situ ATR-FTIR spectroscopy for evidencing the adsorption mechanism of ammonium on a pinewood-derived biochar

Biochar is used as a soil amendment for improving soil health. Biochar is known to possess high adsorption capacity for nutrient ions, especially toward NH₄⁺. However, there is limited information regarding the direct binding mechanisms of NH₄⁺ on biochar. Although few infrared spectroscopic studies were conducted by characterizing the biochar solid phase, no research was reported, in which the adsorption mechanism was deciphered in the presence of water. Here we report NH₄⁺ adsorption mechanism on pinewood biochar using in situ attenuated total reflectance Fourier transform infrared spectroscopy. The results indicated that NH₄⁺ adsorbed strongly on pinewood biochar at pH 8.5 and less at pH 5.5 as revealed by infrared band shifts and symmetry change. Macroscopic adsorption envelope corroborated the spectroscopic finding by resulting in higher adsorption capacity with increasing pH.